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Pythagorean by Refection

Francis Su was here last week, sharing some of his Fun Math Facts. I loved this Pythagorean Proof using only reflections. Also, non-squares for the similar figures on each side. This is soooo much more organic than squares. Also generalizes to obtuse and acute more easily. Now I’m wondering abbout the Law of Cosines... I can’t find the source of this one, though. So how could you get students to know that if you’ve proved it for any similar objects, you’ve proved it for all similar objects?
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